The joint detection of gravitational waves (GWs) and electromagnetic (EM) radiation from the binary neutron star merger GW170817 ushered in a new era of multi-messenger astronomy. Joint GW-EM observations can be used to measure the parameters of the binary with better precision than either observation alone. Here, we use joint GW-EM observations to measure the viewing angle of GW170817, the angle between the binary's angular momentum and the line of sight. We combine a direct measurement of the distance to the host galaxy of GW170817 (NGC 4993) of 40.7±2.36 Mpc with the Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo GW data and find that the viewing angle is 32−13+10±1.7 degrees (90% confidence, statistical, and systematic errors). We place a conservative lower limit on the viewing angle of ≥13∘, which is robust to the choice of prior. This measurement provides a constraint on models of the prompt γ-ray and radio/X-ray afterglow emission associated with the merger; for example, it is consistent with the off-axis viewing angle inferred for a structured jet model. We provide for the first time the full posterior samples from Bayesian parameter estimation of LIGO/Virgo data to enable further analysis by the community.
@article{arxiv.1804.04179,
title = {Measuring the viewing angle of GW170817 with electromagnetic and gravitational waves},
author = {Daniel Finstad and Soumi De and Duncan A. Brown and Edo Berger and Christopher M. Biwer},
journal= {arXiv preprint arXiv:1804.04179},
year = {2018}
}